1. Field of the Invention
The present invention relates to a voltage regulator provided with an inrush current protection circuit and more particularly to an inrush current protection circuit that controls an inrush current by restricting a fluctuation at the gate of an output driver so as to restrain an inrush current into an output capacitor occurring at a startup.
2. Description of the Related Art
A conventional inrush current protection circuit will be described. FIG. 3 is a circuit diagram of a conventional constant-voltage circuit. The constant-voltage circuit is composed of a constant-voltage source 401 and a soft start circuit, which is an inrush current protection circuit. The soft start circuit has a comparator 404, a delay circuit 412, a constant-current source 407, a capacitor 408, a resistor 403, and switches 402, 410 and 411.
The contact point of the constant-current source 407 and the capacitor 408 is connected to an output terminal 101 of the constant-voltage circuit. The output terminal 101 is connected to a non-inverting input terminal of the comparator 404, and an output terminal of the constant-voltage source 401 is connected to an inverting input terminal of the comparator 404 through the intermediary of an offset voltage 405. An output terminal of the comparator 404 is connected to the switch 402, the constant-current source 407, and the delay circuit 412. An output terminal of the delay circuit 412 is connected to the switch 411.
The capacitor 408 is charged by receiving constant current Ic from the constant-current source 407. The comparator 404 compares the voltage obtained by subtracting the predetermined offset voltage 405 from an output voltage of the constant-voltage source 401 and the voltage at the contact point of the constant-current source 407 and the capacitor 408, and issues an output voltage based on the result of the comparison. The output voltage of the comparator 404 controls the switch 402, the constant-current source 407, and the switch 411 through the delay circuit 412. When the switch 402 is turned on, the capacitor 408 is charged by the constant-voltage source 401 through the resistor 403 on the basis of an RC time constant. After predetermined time elapses since a output voltage is received from the comparator 404, the delay circuit 412 turns the switch 411 on. When the switch 411 is turned on, the output voltage of the constant-voltage source 401 is directly output to the output terminal 101.
The operation of the conventional constant-voltage circuit will now be described. In the state wherein the switch 410 is on, the constant-voltage circuit is not in operation and the output voltage at the output terminal 101 is 0 volt. When the switch 410 is turned off, the constant-voltage circuit is actuated. The constant-current source 407 supplies the constant current Ic to start charging the capacitor 408 with the constant current. At this time, the output voltage at the output terminal 101 linearly rises according to the constant current Ic and the capacitance of the capacitor 408. If the voltage charged in the capacitor 408 exceeds the voltage, which is obtained by subtracting the offset voltage 405 from the voltage of the constant-voltage source 401, then an output signal of the comparator 404 is inverted. This causes the switch 402 to turn on and the constant-current source 407 to stop and the delay circuit 412 to be actuated. When the constant-current source 407 stops its operation, the capacitor 408 is charged with the output voltage of the constant-voltage source 401 through the resistor 403.
The moment the switch 411 turns on after the elapse of the predetermined time since the delay circuit 412 was actuated, the output voltage of the constant-voltage source 401 immediately reaches the output voltage at the output terminal 101. As described above, the output voltage at the output terminal 101 of the constant-voltage circuit gradually increases, thus allowing the output terminal 101 of the constant-voltage circuit to be protected from an inrush current (refer to, for example, FIG. 2 in patent document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-56843
However, the conventional art has been posing a problem in that changing from a soft start phase over to a constant-voltage output phase by a switch inconveniently causes discontinuity in a linearly increasing output voltage. There has been another problem in that the need for a comparator and a delay circuit inevitably results in a larger circuit scale.